For GTK+ dialogs, Emacs uses a widget named emacs-dialog, of
class GtkDialog. For file selection, Emacs uses a widget named
emacs-filedialog, of class GtkFileSelection.

Because the widgets for pop-up menus and dialogs are free-standing
windows and not “contained” in the Emacs widget, their GTK+
absolute names do not start with ‘Emacs’. To customize these
widgets, use wildcards like this: